A salty ocean more than 100 kilometers deep might lurk beneath Pluto’s icy heart, a new study suggests. The buried reservoir could have helped tip the dwarf planet over at some point in its past, bringing the heart-shaped region in line with gravitational forces from Charon, Pluto’s largest moon.

A subsurface ocean isn’t a new idea; researchers proposed the possibility in March to explain the alignment between Charon and Sputnik Planum — the frozen impact basin that forms the left side of Pluto’s heart. Brandon Johnson, a planetary scientist at Brown University, and colleagues ran computer simulations to estimate the thickness of the putative sea. They report their results online September 19 in Geophysical Research Letters.